Re: To take advantage of Turbo Boost the four threads should be allocated to two cores, and this will allow taking advantage of TurboBoost. However in some situations having one thread per core,gives each thread exclusive use of the L1 cache and all the other ALU and other Core resources.
Performance wise, I think you are always better off going with more cores, rather than more frequency, but it all depends on how well the application can take advantage of multiple threads. For those applications that are not well threaded, TurboBoost seems to be a means to have the best of both worlds.